1. Field of the Invention
The present invention relates to a transmission electron microscope (TEM) equipped with an energy filter.
2. Description of the Related Art
A transmission electron microscope (TEM) equipped with an energy filter has been heretofore known. An energy filter has a function of energy-dispersing electrons transmitted through a specimen. A transmission electron microscope equipped with an energy filter is capable of selecting only those electrons transmitted through a specimen which have a certain energy and imaging them. Hence, the contrast and resolution of the image can be improved. Also, this kind of instrument can gain an energy loss spectrum and a two-dimensional distribution of elements constituting the specimen.
An example of the structure of a transmission electron microscope equipped with an energy filter is shown in FIG. 2. Shown in this figure are an electron gun 1, a system of condenser lenses 2, a specimen stage 3 including a specimen holder, an objective lens 4, an intermediate lens system 5, an energy filter 6, an energy selecting slit 7, a projector lens system 8, and an imaging/recording unit 9.
Note that the xe2x80x9cprojector lensxe2x80x9d referred to herein means a lens located between the energy filter 6 and the imaging/recording unit 9. The xe2x80x9cintermediate lensesxe2x80x9d mean one or more lenses positioned between the objective lens 4 and the energy filter 6. The imaging/recording unit 9 is made of a fluorescent screen, a photography device, or a TV camera.
The energy filter 6 acts to energy-disperse incident electrons. Various kinds of energy filters are known. For example, various energy filters each made up of two or more electromagnets are known as omega, alpha, gamma, and mandolin type filters according to their orbital geometry of electrons within the energy filter 6. Furthermore, a Wien filter making use of electric and magnetic fields that are made to overlap with each other is known. Any type can be used as the energy filter 6 shown in FIG. 2. For convenience, an omega filter is used below.
One known intermediate lens system, such as the intermediate lens system 5, is made up of a single lens stage. Another known intermediate lens system is made up of two stages of lenses. A further known intermediate lens system consists of three stages of lenses. In the illustrated structure, the intermediate lens system is made of three stages of lenses. Each stage consists of one or more lenses. The projector lens system 8 is made of one or more stages of lenses, and each stage consists of one or more lenses.
Four points or locations are defined within the energy filter 6. For the description given below, these four locations are described by referring to FIG. 3. The omega filter is made up of four electromagnets 10, 11, 12, and 13 as shown in FIG. 3. The electromagnets 10 and 13 are vertically symmetrical with respect to a position indicated by the dot-and-dash line. Similarly, the electromagnets 11 and 12 are vertically symmetrical with respect to the position indicated by the dot-and-dash line. In FIG. 3, the central orbit of the electron beam in the omega filter is indicated by the broken line O.
The aforementioned four points are defined in FIG. 3, that is, these are an incident crossover point A, an incident image plane B, an exit image plane C, and an exit crossover point D.
The position of the exit image plane C is so set that electrons having different energies are focused at the same position. This plane C is also known as an achromatic image plane. On the other hand, the exit crossover point D is also termed an energy dispersive plane. The slit 7 is positioned at this exit crossover point D.
These four points A, B, C, and D are defined strictly according to the design of the energy filter, and are important optical positions where they are used as components of the TEM equipped with energy filter. In order to minimize the imaging distortion of the energy filter and to secure the same imaging function as used in an ordinary TEM, these four points must be defined strictly. These four points A-D are defined in any structure including an omega filter.
The operation of each part of the TEM equipped with energy filter is now described.
(1) Action of the intermediate lens system 5 in the TEM equipped with energy filter:
The intermediate lens system 5 acts to vary either the magnification of a TEM image that is a magnified image of a specimen or the camera length for a diffraction pattern.
The intermediate lens system also serves to bring an image and a diffraction pattern formed by the objective lens 4 into positions defined in the energy filter 6. In particular, where electrons having a desired energy are selected by the slit 7 and an image is observed, the intermediate lens brings the diffraction pattern formed at the back focal plane of the objective lens 4 to the incident crossover point A. Also, the intermediate lens system brings an image formed by the objective lens 4 to the incident image plane B. Where electrons having a desired energy are selected by the slit 7 and a diffraction pattern is observed, the intermediate lens system brings an image formed by the objective lens 4 to the incident crossover point A. Also, the intermediate lens brings a diffraction pattern formed at the back focal plane of the objective lens 4 to the incident image plane B.
In summary, the intermediate lens system 5 performs the following three actions:
(i) The intermediate lens system 5 brings an image or a diffraction pattern that is conjugate with the image to the incident crossover point A defined in the energy filter 6.
(ii) The intermediate lens system 5 brings a diffraction pattern or an image that is conjugate with the diffraction pattern to the incident image plane B defined in the energy filter 6.
(iii) In the imaging mode, the intermediate lens system 5 can vary the magnification in minute steps over a wide range. In the diffraction mode, the intermediate lens system 5 can vary the camera length in minute steps over a wide range.
To perform these three actions, it is necessary to fabricate the intermediate lens system 5 from three stages of lenses, because any one lens cannot alone satisfy each of the three conditions described above. Three stages of lenses are so adjusted as to complement each other in satisfying the three conditions.
(2) Action of the energy filter 6 in the TEM equipped with energy filter:
Electrons emitted by the electron gun 1 are focused onto the specimen by the system of condenser lenses 2. If the thickness of the specimen is only several micrometers or less, the electrons are transmitted through the specimen. During this process, the electrons interact with atoms and electrons constituting the specimen, whereby energy loss takes place. The energy loss is not uniform for all electrons but has a probability distribution that is characteristic of the specimen.
In the transmission electron microscope equipped with energy filter, electrons are energy-dispersed by the energy filter 6. The energy loss distribution is analyzed. Thus, the state of electrons, such as free electrons and bound electrons within the specimen, can be known. Furthermore, if the slit 7 is placed in the exit crossover point D in the energy filter 6, and if only electrons suffering from certain energy loss are selected by the slit 7 and imaged, then a two-dimensional distribution image of energy loss in the specimen can be obtained. This can be applied to analysis of elements constituting the specimen or can be used to improve the image contrast.
In the imaging mode as described above, the intermediate lens system 5 forms a diffraction pattern at the incident crossover point A. Also, the intermediate lens system 5 forms an image at the incident image plane B. The diffraction pattern at the incident crossover point A is focused at the exit crossover point D by the electron-refracting action of the energy filter 6. The image at the incident image plane B is focused at the exit image plane C.
In the diffraction mode, the intermediate lens system 5 forms an image at the incident crossover point A. Also, the intermediate lens system 5 forms a diffraction pattern at the incident image plane B. The image at the incident crossover point A is focused at the exit crossover point D by the electron-focusing action of the energy filter 6. The diffraction pattern at the incident image plane B is focused at the exit image plane C.
(3) Action of the projector lens system 8 in the TEM equipped with energy filter:
An image or a diffraction pattern is focused at the exit imaging plane C in the energy filter 6. An energy loss spectrum is focused at the exit crossover point D. Which of them is projected onto the image-receiving surface of the imaging/recording unit 9 depends on whether the projector lens system 8 is focused at the exit image plane C or the exit crossover point D. If the projector lens system 8 is focused at the exit image plane C, then the image or diffraction pattern is created or recorded on the imaging/recording unit 9. If the projector lens system 8 is focused at the exit crossover point D, then the energy loss spectrum is formed or recorded on the imaging/recording unit 9.
In this way, the projector lens system 8 acts to select the imaging mode, i.e., whether the image/diffraction pattern focused by the energy filter 6 or the energy loss spectrum is finally focused.
All of the condenser lenses 2, objective lens 4, intermediate lenses 5, and projector lenses 8 are magnetic lenses. When a magnetic lens focuses an image, electrons are rotated about the optical axis. Let the center axis along the optical axis be the Z-axis. When electrons move from a position indicated by Z=z1 to a position indicated by Z=z2 across the magnetic lens, the electrons rotate through an angle xcfx86 given by:                     φ        =                                            e                              8                ⁢                                  xe2x80x83                                ⁢                                  mU                  *                                                              ⁢                                    ∫                              z                1                                            z                2                                      ⁢                                          B                ⁡                                  (                  z                  )                                            ⁢                              ⅆ                z                                                                        (        1        )            
where e is the electron charge of an electron, m is the mass of an electron, U* is the relativistically modified accelerating voltage, and B(z) is the strength of the magnetic field at Z-coordinate z. Note that:       U    *    =      E    ⁡          (              1        +                  e          ⁢                      E                          2              ⁢                              m                0                            ⁢                              c                2                                                        )      
where E is the accelerating voltage, e is the elementary charge, m0 is the electron rest mass, and c is the velocity of light.
Rotation caused by the focusing action of the intermediate lens system 5 is now discussed. Since the intermediate lens system 5 is sufficiently remote from the specimen, the lens action of the intermediate lens system 5 does not reach the specimen. Accordingly, with respect to the intermediate lens system 5, the z1 and z2 included in Eq. (1) above can be taken as xe2x88x92∞ and +∞, respectively. In this case, Eq. (1) can be simplified into the form:                     φ        =                              0.186            ·                          NI                                                U                  *                                                              ⁢                      (            radian            )                                              (        2        )            
where N is the number of turns of wire on the coil that causes the lens to produce a magnetic field and I is the excitation current (hereinafter also referred to as the lens current) fed to the coil. Therefore, it can be seen that the angle xcfx86 is in proportion to the product of the number of turns of the wire on the coil and the lens current fed to it. The polarity of the lens current is also taken into account. That is, where a lens current flowing in a direction is taken to have a positive polarity, a lens current flowing in the reverse direction is taken to have a negative polarity. This principle also applies to the following description.
In this way, the angle varies according to the lens current flowing through the lens. That is, when the lens conditions are varied, the image or diffraction pattern is rotated. In the TEM equipped with energy filter, the intermediate lens system 5 has only three stages and so it is possible to perform only the actions (i), (ii), (iii) described above; it has been impossible to control the angle.
Therefore, if the magnification of the image or the camera length for the diffraction pattern is varied, and if the mode is changed from imaging mode to diffraction mode or vice versa, rotation upsets the angular relation between the finally obtained image or diffraction pattern and the specimen.
One countermeasure against this problem is to measure the angle of rotation when the mode is varied from imaging mode to diffraction mode or vice versa. Where the magnification or the camera length is modified, the angle is previously measured in each step. However, it is very cumbersome to analyze plural images of different magnifications and diffraction patterns. Consequently, the analysis tends to produce erroneous results.
Accordingly, it is an object of the present invention to provide a transmission electron microscope (TEM) equipped with energy filter that permits the angular position of an image or diffraction pattern to be controlled when lens conditions are modified.
The above-described object is achieved by a TEM equipped with energy filter comprising an objective lens system, a first intermediate lens system, a second intermediate lens system, a third intermediate lens system, a fourth intermediate lens system, an energy filter, and a projector lens system. Each lens system consists of at least one lens. This microscope is characterized in that an excitation current supplied to each of the lenses is controlled to prevent rotation of a created image.
In one aspect of the present invention, if the mode of operation of the microscope is varied from the imaging mode to the diffraction mode or vice versa, if the magnification is modified in the imaging mode, or if the camera length is changed in the diffraction mode, the total sum of the products of the numbers of turns of wire on coils of the lenses of the various systems (i.e., the objective lens system, the first intermediate lens system, the second intermediate lens system, the third intermediate lens system, the fourth intermediate lens system, and the projector lens system) and their respective excitation currents is kept constant.
In another aspect of the present invention, each value of the products of the numbers of turns of wire on the coils of the objective lens system and their respective excitation currents is multiplied by a coefficient k.
In a further aspect of the present invention, there is further provided a look-up table in which the excitation currents supplied to the lenses of the various systems are stored for plural modes of operation, various values of magnification, and various values of camera length.
Other objects and features of the invention will appear in the course of the description thereof, which follows.